1. Field of the Invention
This invention relates to a novel polyethylene composition. More particularly, the invention relates to a polyethylene composition which is excellent in melt elasticity such as melt tension and die swell ratio, flow characteristics such as workability, and mechanical properties such as impact resistance and tensile strength, and which has a wide molecular weight distribution. Especially, the polyethylene composition of the invention excels in mechanical properties at low temperatures having large values in melt tension and die swell ratio, so that the composition is suitable to use in making large hollow articles such as gasoline storage tanks and extruded articles such as large-diameter pipes.
2. Description of the Prior Art
In the conventional art, there are proposed methods to widen the molecular weight distribution of an ethylene-.alpha.-olefin copolymer in order to improve the flow characteristics (e.g. Japanese Laid-Open Patent Publication No. Sho 57-21409 and Japanese Patent Publication No. Sho 63-7741). Even when the molecular weight distribution is widened like in the proposed methods, the melt elasticity and mechanical properties, particularly low temperature mechanical properties, cannot be improved but they are made worse to the contrary.
Furthermore, concerning the improvement in mechanical properties and flow characteristics of ethylene-.alpha.-olefin copolymers which are composed of high molecular weight components and low molecular weight components, it was tried to improve not only the mechanical properties and flow characteristics but also the environmental stress cracking resistance (ESCR) by specifying the degree of short chain branching in high molecular weight components and by introducing a large quantity of short branched chains into the high molecular weight components (Japanese Laid-Open Patent Publication No. Sho 54-100444 and Japanese Patent Publication No. Sho 64-7096). The methods are, however, still unsatisfactory in view of the mechanical properties, particularly the low temperature mechanical properties and melt elasticity.
Still furthermore, it is proposed in Japanese Laid-Open Patent Publication No. Hei 2-305811 that the catalyst and conditions for two-stage polymerization are specified in order to improve the impact resistance, ESCR, and pinch-off adhesion. In this method, however, the ESCR and melt elasticity can be improved to some extent but the improvement in mechanical properties, particularly, the low temperature mechanical properties and melt elasticity are not sufficiently improved.
Besides the above, polyethylene compositions for use in blow molding having improved draw-down resistance, die swell ratio and ESCR are disclosed in Japanese Laid-Open Patent Publication Nos. Sho 59-89341 and Sho 60-20946. Furthermore, three-stage polymerization methods as an improvement of two-stage polymerization method are disclosed in Japanese Patent Publication No. Sho 59-10724 and Japanese Laid-Open Patent Publication Nos. Sho 62-25105, 62-25106, 62-25107, 62-25108, and 62-25109. Also in these references, although the improvement in melt elasticity and flow characteristics is attained to some extent, the improvement is not satisfactory yet particularly in the mechanical strength at low temperatures.